Magnetic sensor system and method

ABSTRACT

A sensor system method of production includes forming first and second structures of the magnetoresistive system, heating the first and second structures, applying a magnetic field in a reference direction to the first and second structures, and cooling the first and second structures to fix a reference magnetization in the first and second structures in the reference direction. The structures are heated to near or above a blocking temperature, whereby the shape anisotropy of the first structure forces the reference magnetization to rotate into a first new orientation and the shape anisotropy of the second structure forces the reference magnetization to rotate into a second new orientation whereby the reference magnetization in the first and second structures rotate in opposite directions. The rotated reference magnetizations of the first and second structures are pinned in the respective new orientation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Utility Patent Application is a continuation application of U.S.application Ser. No. 12/269,168, filed Nov. 12, 2008, which isincorporated herein by reference.

BACKGROUND

The resistance of sensors based on Magnetoresistance (MR), such as GiantMagnetoresistance (GMR) or Tunnel Magnetoresistance (TMR) sensors,varies when an external magnetic field is applied to the sensor device.Magnetoresistance angle sensors are widely used in non-contact angularposition sensors in harsh environments like in automobiles or industry.In general, these sensors are relatively insensitive to wear andcontamination.

Magnetoresistance “spin valve” sensors typically have two magneticlayers separated by a non-magnetic layer. One of the two magnetic layersis pinned so that it has a fixed orientation in a reference direction,while the other layer is a free layer that changes orientation inresponse to an external magnetic field.

GMR angle sensors, for example, use bridge configurations with oneresistor that reduces its resistance and another than increases itsresistance when a certain physical magnitude is applied. For GMRspin-valve sensors, this can be accomplished by using GMR sensorresistors for the legs of the bridge that are magnetized in twodifferent reference magnetization directions. To fix the referencemagnetization direction, the structure is heated, an external magneticfield is applied then the structure is cooled to fix the magnetizationdirection.

For monolithically integrated sensors requiring multiple magnetizationdirections, this process must be repeated for each magnetizationdirection. To prevent over-writing the previously fixed magnetizations,typically a localized magnetization process is applied, such as by alocal heating (for example, by monolithically integrated heating stripesor a laser spot) within an external magnetic field then cooling to fixthe magnetization direction. This process has to be conducted for eachmagnetization direction, which adds complexity and cost to themanufacturing process.

SUMMARY

Embodiments of a sensor system and production method are disclosed. Insome embodiments, a method of production includes forming first andsecond structures of the magnetoresistive system, heating the first andsecond structures, applying a magnetic field in a reference direction tothe first and second structures, and cooling the first and secondstructures to fix a reference magnetization in the first and secondstructures in the reference direction. The first and second structuresare heated to near or above a blocking temperature, whereby the shapeanisotropy of the first structure forces the reference magnetization inthe first structure to rotate into a first new orientation and the shapeanisotropy of the second structure forces the reference magnetization inthe second structure to rotate into a second new orientation whereby thereference magnetization in the first structure and the referencemagnetization in the second structure rotate in opposite directions. Therotated reference magnetizations of the first and second structures arepinned in the respective new orientation.

In other embodiments, a sensor system includes a movable magnet havingan axis of rotation, a first magnetoresistive sensor resistor situatedunder the axis of rotation, the first magnetoresistive sensor resistorincluding a pinned magnetic layer having a fixed orientation in areference magnetization direction, and being configured such that theresistance of the first sensor resistor changes in response to an angledefined between the reference magnetization direction and a magneticfield. A plurality of second magnetoresistive sensor resistors surroundthe first magnetoresistive sensor resistor in inhomogeneous portions ofthe magnetic field, and are configured to provide a differential signal,with each of the second magnetoresistive sensor resistors including apinned magnetic layer having a fixed orientation in the referencemagnetization direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present invention and are incorporated in andconstitute a part of this specification. The drawings illustrate theembodiments of the present invention and together with the descriptionserve to explain the principles of the invention. Other embodiments ofthe present invention and many of the intended advantages of the presentinvention will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIG. 1 conceptually illustrates an embodiment of a sensor system formeasuring angular position.

FIG. 2 conceptually illustrates an example of a magnetoresistive (MR)sensor resistor.

FIG. 3 illustrates a process for pinning a reference magnetizationdirection for an MR sensor resistor.

FIG. 4 illustrates an embodiment of an MR sensor system using arectangular magnet.

FIG. 5 illustrates an embodiment of an MR sensor system using a ringmagnet.

FIG. 6 illustrates an example of signals output by a system according toFIG. 4 or 5.

FIG. 7 illustrates another embodiment of an MR sensor system.

FIG. 8 illustrates the system of FIG. 7 with reference magnetizationdirections rotated.

FIG. 9 illustrates an example of signals output by systems according toFIGS. 7 and 8.

FIG. 10 illustrates differential output signals according to the systemaccording to FIG. 8.

FIG. 11 conceptually illustrates an embodiment of an MR resistor stack.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments of the present invention can be positioned ina number of different orientations, the directional terminology is usedfor purposes of illustration and is in no way limiting. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

FIG. 1 illustrates an embodiment of a sensor system 100 for measuringangular position. The system 100 includes a sensor 112 made up of aplurality of magnetoresistance (for example, GMR) sensor resistors. Amovable magnet 110 is situated adjacent the sensor 112. The magnet 110may be fastened to a rotatable shaft, for example. The magnet 110establishes a magnetic field, so that when the magnet 110 rotates, themoving magnetic field causes the resistance of the sensor resistors ofthe sensor 112 to vary. The changing resistance is used to determine theangular position of the magnet 110, and thus the position of the shaftor other rotating device to which the magnet is attached. In theillustrated embodiment, the output of the sensor 112 is provided to aprocessor 114 that is programmed to determine the angular position ofthe rotatable magnet 110 based on the varying resistances.

In typical known sensors, the sensor resistors are configured in twobridges, one with magnetization in an x direction and one withmagnetization in a y direction. The x bridge provides a signalproportional to the cosine of the angle between the direction of themagnetic field of the magnet 110 and a reference magnetizationdirection, and the y senor provides a sine signal proportional to thesame angle. Using both signals, the angle measurement is unambiguous.Further, the sensor resistors diagonally situated in each bridge aremagnetized counter-directional in order to achieve the maximumsensitivity. Thus, known sensors typically require four differentmagnetization directions.

FIG. 2 illustrates an example of an MR sensor resistor 200 included inthe sensor 112. Each of the sensor resistors 200 of the sensor 112 has apinned magnetic layer 202 that has a fixed orientation in a referencemagnetization direction 204. A nonmagnetic spacer layer 206 separates afree magnetic layer 208 and the pinned layer 202. FIG. 3 illustrates aprocess for pinning the reference magnetization direction 204. In theillustrated embodiment, the pinned layer 202 includes anantiferromagnetic layer 210 and a ferromagnetic layer 212. In someembodiments, the pinned layer 202 further includes a spacer layer and areference layer. In FIG. 3 a, there is no specific magnetization of theferromagnetic layer 212.

In general, the sensor resistor structures are formed on a commonsubstrate and the magnetization is accomplished by applying an externalmagnetic field in the desired direction, heating the sensor element tobe magnetized in this direction and cooling it. In FIG. 3 b, heat isapplied to the structure and a magnetic field is applied in thereference direction 204. In some embodiments, the antiferromagneticlayer 210 is formed from platinum manganese (PtMn) and is heated to atemperature between about 300-400° C., for example. In FIG. 3 c, thestructure is cooled such that the exchange bias aligns the surface ofthe antiferromagnetic layer 212, resulting in pinning the ferromagneticlayer 212 in the reference direction 204 as illustrated in FIG. 3 d.

For sensor systems that require different magnetization directions forthe individual sensor resistors, a separate heating process is requiredfor pinning each magnetization direction. Moreover, the elements have tobe magnetized without heating elements that have been previouslymagnetized, as this would overwrite the magnetization direction.Therefore, the magnetization direction is typically set using localheating in combination with a local/global magnetic field. In accordancewith aspects of the present disclosure, embodiments are disclosed thatrequire a single heating process to set the reference magnetizationdirection for all sensor resistors on a common substrate of a MR sensorsystem.

FIG. 4 illustrates an embodiment of a sensor system 300 that uses aplurality of MR sensor resistors magnetized in the same referencedirection. The sensor system 300 includes a first MR sensor resistor 301that has a pinned magnetic layer with a fixed orientation in a referencemagnetization direction 204 as described above. The first sensorresistor 301 is situated adjacent a rotatable magnet 110 in a homogenousportion of the magnetic field established by the magnet 110. In otherwords, the first sensor resistor 301 is positioned in a location wherethe magnetic field lines 312 are relatively straight and extend in thesame direction.

In this position, the resistance of the first sensor resistor 301changes in response to the magnetic field established by the magnet 110.The resistance varies between a minimum and a maximum depending on theangle between the reference magnetization direction 204 and the magneticfield, following a sine function.

The group of second sensor resistors 302 are configured to provide adifferential signal. Each of the second sensor resistors 302 also has apinned magnetic layer with a fixed orientation in the same referencedirection 204. Thus, the reference magnetization of all of the resistors301, 302 can be set using a single heating process.

The second resistors 302 are situated in a region 314 of the magneticfield that is inhomogeneous. The locations of those resistors are chosenin a way that the difference has minima and maxima in angle positionsthat are 90° phase shifted versus the sine of the single first resistor301. Generally, the resistance of the second resistors 302 does notfollow a sine function, however both signals together deliverunambiguous information about the angle between the axis of the magnet110 and the magnetization direction 204 of the resistors 301,302.

In the embodiment illustrated in FIG. 4, the first sensor resistor 301is situated under the rotation axis of the magnet 110. The second sensorresistors include four resistors 302 a, 302 b, 302 c, 302 d generallysurrounding the first sensor resistor 301 and positioned in aninhomogeneous portion of the magnetic field—the portion under the straymagnetic field 314 where the magnetic field lines are curved.

In general, for an MR sensor a magnetic field in direction of thereference magnetization lowers the resistance of the MR sensor resistor,and a field in opposite direction increases it. In FIG. 4 a, the longaxis of the magnet 110 is oriented perpendicular to the referencedirection 204. In this position, the stray magnetic field lines 314 areoriented generally opposite the reference direction 204 for the firstand third sensor resistors 302 a and 302 c and therefore increase theresistance of these sensor resistors. For the second and fourth sensorresistors 302 b and 302 d, the stray field lines 314 are generallyoriented in the same direction as the reference direction 204, and thus,the resistance is decreased.

In FIG. 4 b, the magnet 110 has rotated 90° such that it is orientedparallel with the reference direction 204 of the sensor resistors 301,302. In this position, the stray magnetic field lines 314 are generallyoriented in the reference direction 204, thus decreasing the resistanceof all of the second sensor resistors 302 a,302 b,302 c,302 d. FIG. 4 cillustrates the system 300 with the magnet 110 at about a 45° anglerelative to the reference direction 2004. For the first and third sensorresistors 302 a, 302 c of the second resistor group, the stray magneticfield lines 314 have a negligible effect on the resistance of thesesensor resistors 302 a, 302 c because the stray field lines 314 extendin both directions. In the region of the second and fourth sensorresistors 302 b,302 d, the field lines 314 extend generally in the samedirection as the reference direction 204 and thus, decrease theresistance of the second and fourth sensor resistors 302 b,302 d.

FIG. 5 illustrates an embodiment of a sensor system 300 where a ringmagnet 111 is used rather than the rectangular magnet 110 illustrated inFIG. 4. In FIG. 5 a, the homogeneous field lines 312 established by thering magnet 111 are oriented perpendicular to the reference direction204. Accordingly, the stray magnetic field lines 114 are orientedgenerally opposite the reference direction 204 for the first and thirdsensor resistors 302 a and 302 c and therefore increase the resistanceof these sensor resistors. For the second and fourth sensor resistors302 b and 302 d, the stray field lines are generally oriented in thesame direction as the reference direction 204, and thus, the resistanceis decreased.

In FIG. 5 b, the ring magnet 111 is rotated 90° such that the fieldlines 312 are parallel with the reference direction 204 of the sensorresistors 301, 302. In this position, the stray magnetic field lines 314are generally oriented in the reference direction 204, thus decreasingthe resistance of all of the second sensor resistors 302 a,302 b,302c,302 d.

FIG. 6 illustrates an example of signals from the sensor resistors 301,302. As noted above, the first sensor resistor 301 outputs a sinusoidalsignal 320 in response to the rotating magnet. The second sensorresistors 302 are configured to provide differential signals, where thecurve 322 of the second resistors 302 a, 302 b, 302 c, 302 d iscalculated by combining the resistances according to (302 a-302 b)+(302c-302 d), for example. The functions (302 d-302 b) and (302 a-302 c)should always be 0, which can be used for adjustment of the magnetposition and for self testing of misalignment or asymmetrical damage ofthe sensors.

In some embodiments, the angle is determined using a look up table thatcorrelates the signals of the first and second resistors 301, 302 withangular positions. In other embodiments, the arctan of the quotient ofboth signals is calculated. The difference of the second sensor resistorgroup 302 is not an exact cosine function. Rather, the result is adisturbed angle and is linearized. In some embodiments, the arcsin ofthe first sensor resistor 301 is calculated and the correct region onthe angle axis is identified based on the sign of the differentialsignal from the second resistors 302.

FIG. 7 illustrates another embodiment of a sensor system 400. Thisembodiment makes use of the shape anisotropy in narrow MR stripestructures (for example, less than 5 μm in width). The sensor systemincludes first and second MR sensor resistors 401, 402. The first andsecond sensor resistors 401,402 each have a pinned referencemagnetization direction that is set, for example, with a heating processdescribed above. As with the earlier-disclosed embodiments, thereference magnetization direction of the first and second sensorresistors 401,402 is pinned using a single heating process.

With known MR sensors, the reference magnetization direction istypically perpendicular to the stripe length axis. In the illustratedembodiment, the sensor resistors 401,402 each have a length axis 410.The first sensor resistor 401 has its length axis 410 oriented by 90°+anangle Φ, where Φ<90°, relative to the axis of the referencemagnetization direction 204. The second sensor resistor 402 has itslength axis 410 oriented by 90°−Φ relative to the referencemagnetization axis 204. In some embodiments, Φ is between 5° and 85°. Insome embodiments, the value of the angle Φ is not exactly the same forthe first and second resistors 401,402 due to manufacturing tolerances,etc. Electrically/magnetically a small difference (for example, +/−4°)between the angle Φ in the first and second resistors does not make asignificant difference.

An asymmetry between the first and second sensor resistors 401,402 isthus introduced. Assuming the same initial reference magnetizationdirection (or axis) 204, the shape anisotropy forces the referencemagnetization for one of the sensor resistors into one direction, andinto the opposite direction for the other sensor resistor. When thesensor 400 is heated near or above the temperature required to pin thereference magnetization direction, or the “blocking temperature” (about300-400° C. for PtMn as the natural antiferromagnet), the referencemagnetization is almost free to be rotated along its anisotropy forcegradient—in opposite directions for the first and second sensorresistors 401,402 as illustrated in FIG. 8.

After cooling down, the new orientation of the reference magnetizations204 a, 204 b is pinned. As a result, two sensor resistors are formedwhich exhibit at least components that are aligned in an anti-parallelmanner to each other. Therefore, by a combination of the two resistorsin a Wheatstone bridge configuration, a differential output signal isobtained.

FIG. 9 is a chart illustrating experimental data with first and secondcurves 421,422 plotting the output signals of the sensor resistors of anon-optimized MR sensor with first and second sensor resistors having atilt angle of +/−45°. The curves 421,422 match for much of range ofmagnetic field angles. After applying 375° C. for one hour, the rotatedreference magnetization directions of the first and second sensorresistors is pinned, such as with the embodiment illustrated in FIG. 8.As a result, an angle of about 35° between the reference magnetizationdirections of the first and second sensor resistors is obtained,resulting in a variation between the curves 431,432 which can be used toprovide a differential output signal used to identify the angularposition of the rotating magnet.

Assuming a Wheatstone bridge configuration with the first and secondresistors 401,402, a differential output signal is generated by arotating magnetic field as illustrated in FIG. 10. Such a signal can beused to implement a sensor for detecting a magnetic field directionwithin a restricted angular range (less than 180° in some embodiments)and with low accuracy requirements. Furthermore, magnetic field strengthcan be detected, suitable for use as a proximity switch, for example.

FIG. 11 schematically illustrates MR resistor stack 440 suitable for usein the embodiment illustrated in FIGS. 8 and 9. It has anantiferromagnet layer 442 that includes a natural antiferromagnet layer444 formed from IrMn or PtMn, for example. A ferromagnetic layer, orpinned layer 446 is situated over the natural antiferromagnet 444 and byprocess a very stable magnetization direction. A non-magnetic spacerlayer 448 separates the ferromagnet layer 446 from anantiferromagnetically coupled reference layer 450. The antiferromagneticcoupling is managed by the thickness of the spacer layer 448. Theferromagnetic layer 446 and the reference layer 450 together form anartificial antiferromagnet 442 that enhances the stability againstexternal magnetic fields. Another spacer layer 452 separates thereference layer 450 from a free layer 454. The variation of themagnetization angles between the free layer 454 and the reference layer450 provides the spin valve effect.

For proximity switch implementations, a certain coupling between thefree layer 454 and the reference layer 450 (either ferromagnetic oranti-ferromagnetic) is necessary in order to provide a reset force forthe free layer magnetization. Such a coupling force can be adjusted bythe thickness of the nonmagnetic spacer layer 452. In some embodiments,the reference layer 450 has a thickness between about 0.5 nm and 10 nm,and the pinned layer 446 has a thickness between about 0.5 nm and 10 nm.

The disclosed embodiments allow the formation of a MR sensor systemusing a single heating process such as that disclosed and described inconjunction with FIG. 3 to pin the reference direction of a plurality ofsensor resistor structures. Generally, the plurality of magnetoresistivesensor resistor structures are formed on a substrate, configured asdisclosed in the embodiments described above, for example. The resistorstructures are heated (about 300-400° C. for PtMn) and a magnetic fieldis applied in a reference direction. A furnace or a rapid thermal annealprocess is used for heat application in some embodiments. The structureis then cooled to fix the reference magnetization direction for each ofthe sensor resistor structures. Thus, with a single heating process, theresistor structures can be configured to output a differential signalthat varies in response to an external magnetic field.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A method for producing a magnetoresistive system,the method comprising: forming first and second structures of themagnetoresistive system; heating the first and second structures;applying a magnetic field in a reference direction to the first andsecond structures; cooling the first and second structures to fix areference magnetization in the first and second structures in thereference direction; heating the first and second structures near orabove a blocking temperature, whereby the shape anisotropy of the firststructure forces the reference magnetization in the first structure torotate into a first new orientation and the shape anisotropy of thesecond structure forces the reference magnetization in the secondstructure to rotate into a second new orientation whereby the referencemagnetization in the first structure and the reference magnetization inthe second structure rotate in opposite directions; and pinning therotated reference magnetizations of the first and second structures inthe respective new orientation.
 2. The method of claim 1, wherein eachof the first structure and the second structure is formed as stripe, andwherein the first and second structures each have respective first andsecond length axes in different directions.
 3. The method of claim 2,wherein: the first length axis is oriented by 90 plus a first angle,where the first angle is <90° relative to a reference magnetizationaxis; and the second length axis is oriented by 90 minus a second anglerelative to the reference magnetization axis, where the second angle isabout equal to the first angle.
 4. The method of claim 3, wherein thefirst angle is between 5° and 85°.
 5. A method for producing amagnetoresistive system, the method comprising: forming first and secondstructures of the magnetoresistive system; heating the first and secondstructures; applying a magnetic field in a reference direction to thefirst and second structures cooling the first and second structures tofix a reference magnetization in the first and second structures in thereference direction; heating the first and second structures near orabove a blocking temperature, whereby the shape anisotropy of the firststructure forces the reference magnetization in the first structure torotate into a first new orientation and the shape anisotropy of thesecond structure forces the reference magnetization in the secondstructure to rotate into a second new orientation whereby the referencemagnetization in the first structure and the reference magnetization inthe second structure rotate in opposite directions; and pinning therotated reference magnetizations of the first and second structures inthe respective new orientation, wherein the first and secondmagnetoresistive structures each include: an antiferro magnet layer; apinned magnetic layer situated on the antiferro magnet layer; a firstnon-magnetic spacer layer situated on the pinned magnetic layer oppositethe antiferro magnet layer; a reference magnetic layer situated on thefirst spacer layer opposite the pinned magnetic layer, wherein thereference magnetic layer defines a reference magnetization axis; asecond non-magnetic spacer layer situated on the reference magneticlayer opposite the first spacer layer; and a free magnetic layersituated on the second spacer layer opposite the reference magneticlayer, wherein the free magnetic layer changes orientation in responseto an external magnetic field.
 6. The method of claim 5, wherein thereference layer has a thickness between 0.5 nm and 10 nm.
 7. The methodof claim 5, wherein the pinned layer has a thickness between 0.5 nm and10 nm.
 8. A sensor system, comprising: a movable magnet having an axisof rotation; a first magnetoresistive sensor resistor situated under theaxis of rotation, the first magnetoresistive sensor resistor including apinned magnetic layer having a fixed orientation in a referencemagnetization direction, and being configured such that the resistanceof the first sensor resistor changes in response to an angle definedbetween the reference magnetization direction and a magnetic field; aplurality of second magnetoresistive sensor resistors surrounding thefirst magnetoresistive sensor resistor in inhomogeneous portions of themagnetic field, the second magnetoresistive sensor resistors configuredto provide a differential signal, each including a pinned magnetic layerhaving a fixed orientation in the reference magnetization direction. 9.The sensor system of claim 8, wherein the movable magnet is arectangular magnet.
 10. The sensor system of claim 8, wherein themovable magnet is a ring magnet.
 11. The sensor system of claim 8,wherein the plurality of second magnetoresistive sensor resistorsincludes four resistors.
 12. The sensor system of claim 8, wherein thefirst and second magnetoresistive sensor resistors are situated on acommon substrate.